1. Field of the Invention
The invention relates to bandwidth extending system for an audio signal and a method for generating an audio signal of a wide band from an audio signal whose frequency band is limited to a narrow band by being transmitted through a transmission path such as a telephone line or the like.
2. Description of the Related Art
A band of a telephone line is so narrow to be, for example, 300 to 3400 kHz and a frequency band of an audio signal that is transmitted through the telephone line is limited. Therefore, a sound quality of the conventional analog telephone line is not good. There is also a dissatisfaction about a sound quality of a digital cellular phone.
Various systems for extending an audio band width on the reception side and improving a sound quality have been proposed. Among them, there has been proposed a system such that a narrow band code book in which parameters of a narrow band audio signal derived from patterns of a plurality of audio signals have previously been stored as code vectors and a wide band code book in which parameters of a wide band audio signal derived from the patterns of the same audio signals as those signals have previously been stored as code vectors are prepared, an input signal is analyzed by the narrow band code book, and an audio synthesis is performed by using the wide band code book on the basis of the analysis result, thereby extending an audio band width and improving a sound quality.
That is, as shown in FIG. 6, in case of transmitting an audio signal through a transmission path like a telephone line, a frequency band of the audio signal from a speech side 101 is limited because it is transmitted through a transmission path 102. For example, even if the frequency band of the audio signal from the speech side 101 lies within a range from about 300 Hz to 7000 Hz, so long as it is transmitted via the transmission path 102, a frequency band of an audio signal to be sent to a reception side 103 is limited to a frequency within a range, for example, from about 300 Hz to 3400 Hz.
Therefore, as shown in FIG. 7, a narrow band code book 105 in which parameters of a narrow band audio signal which are derived from patterns of a plurality of audio signals have previously been stored as code vectors and a wide band code book 106 in which parameters of a wide band audio signal obtained from the patterns of the same audio signal have previously been stored in correspondence to the narrow band code book 105 are prepared.
The code books 105 and 106 are formed by, for instance, dividing the same wide band audio signals into frames each having a predetermined length, forming patterns of a plurality of audio signals, and analyzing a spectrum envelope every frame. That is, when the code books are formed, the wide band audio signal is used and the wide band audio signal is divided every predetermined frame. Spectrum envelope information when the wide band audio signal is analyzed as a wide band is stored as code vectors into the wide band code book 106. Spectrum envelope information when the wide band audio signal is band limited to, for example, 300 to 3400 Hz and analyzed is stored as code vectors into the narrow band code book 105.
As spectrum envelope information to be stored in the narrow band code book 105 and wide band code book 106, an LPC cepstrum has been used hitherto. The LPC cepstrum formed is a cepstrum by linear predictive coefficients and is obtained as shown in the following equations (1). ##EQU1##
p: linear predictive degree
In FIG. 7, the narrow band audio signal sent from the speech side 101 to the reception side 103 through the transmission path 102 is first sent to an analyzing circuit 104. In the analyzing circuit 104, the input audio signal is divided every predetermined number of frames and a spectrum envelope is obtained. An output of the analyzing circuit 104 is sent to the narrow band code book 105. In the narrow band code book 105, the spectrum envelope analyzed by the analyzing circuit 104 and the spectrum envelope information stored in the narrow band code book 105 are compared, thereby performing a matching process. An output of the narrow band code book 105 is sent to the wide band code book 106. The spectrum envelope information of the wide band corresponding to the most matched spectrum envelope information in the narrow band code book 105 is read out from the wide band code book 106.
The wide band spectrum envelope information is sent to a synthesizing circuit 107. In the synthesizing circuit 107, the audio signal is synthesized by using the wide band spectrum envelope information read out from the wide band code book 106. Thus the synthesized audio signal becomes the wide band audio signal because it is synthesized by using the wide band code book 106.
As mentioned above, in the conventional audio band width extending system, the LPC cepstrum is used as code vectors. Noises and a pulse train are used as an exciting source when the audio signal is synthesized. In the LPC cepstrum, however, although the auditory distortion and the quantization error relatively coincide, since a logarithm scale is used, importance is attached to a portion of small energy as compared with the case of using a linear scale. An error increases in a portion of a large energy. In case of using the LPC cepstrum in such an audio band width extending system, it is preferable to auditorily suppress a distortion in a vowel sound portion. Therefore, the LPC cepstrum is not always optimum. With respect to the exciting source, although a source that is as close as the LPC residual of the wide band ought to be good, the conventional system using the noises and pulse train is far from it.